metcal STSS's are pretty simple (manual for the "newer" auto-tip off variant says 1992) , and unless they've been seriously abused, will work fine for many more decades to come.
Ah, but there is a caveat. They use parts in the driver stage that are no longer available. So, if the driver FET blows (some VN.... type), you have to do a bit more than just a drop-in replacement. And even the final's are not that easy to come by these days.
While i agree that Metcals usually last a long time (and i have an STSS myself, plus one of my DIY versions), this is something that a potential buyer should be aware of. But in any case, as far as the Metcal 13.56 MHz system is concerned, i think that only a good JBC is comparable in performance. The latter pumping way much more power into the tip to achieve the same result, but anyways, same result nonetheless...
Once you worked with a statin that has such a thermal recovery, you simply don't want to go back, unless forced to.
Oh, and there is the nasty thing about the "older" Metcal stations (STSS, MX500, ...): It will never really switch off the supply. The transformer is always on, the actual switch only switching the secondary, after DC conversion and some regulation.... Be aware of that. But then, you can build your own as well, i have a thread here about that:
https://www.eevblog.com/forum/projects/diy-metcal-13-56-mhz-rf-supply/] [url]https://www.eevblog.com/forum/projects/diy-metcal-13-56-mhz-rf-supply/[/url]
Greetings,
Chris